We are in a similar position to your contributor regarding part time living in Spain.

However, we live in the South in the Andalucia region, and bought a car, Spanish registered, 18 months ago. The buying process was painless, and all that was required was an NIE number(required for any property or large purchase) and a Spanish address. We bought our car privately, and the re registration (matriculation) was done by a local company. It cost around €400. Friends have recently bought from a garage, and that registration was done by the seller. At no time were we asked if we were resident, be it for residential or tax purposes. Hope the above may be of interest, and always stand to be corrected!